Astronauts aboard the International Space Station took a few moments Friday to reflect upon the end of one era and the beginning of another.

During its final flight, space shuttle Discovery delivered the last component of that station, the construction of which is now complete. When it returns to Earth next week, Discovery will be retired.

"We've been very busy during our mission, so mostly we've been spending 95 to 99 percent of our time doing the work and getting the work done," said Steve Lindsey, commander of Discovery. "But there are times, personally, when I've reflected on this being the last mission. I reflect on what a great vehicle it's been, with 39 missions and nearly a year on orbit.

'Frankly sad'

"It's bittersweet, and quite frankly sad, to know that when we land it's the end."

When Discovery departs the station it will fly around the station before heading home.

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With Discovery docked at the station, the station weighs more than 1 million pounds. Astronauts described the interior of the station as cavernous, saying the 12 station and shuttle astronauts can spread out across the orbiting laboratory and not see one another. The habitable volume of the station is larger than that of a Boeing 747.

"To use one of my son's words, It's 'ginormous,' " said Nicole Stott, a Discovery crew member who spent 90 days on the space station in 2009.

During the mission the shuttle crew installed a large cargo container — nicknamed Leonardo - which will give the station a place to store odds and ends.

But this module was also stocked with supplies. To help the station crew members move supplies and unpack the module, mission managers have added two days, so Discovery is now scheduled to land next Wednesday.

"Having the extension of two days of six people to get some of that stuff done before they leave, it just helps maximize our time post-undocking for science and other activities we have to perform on a daily basis," said Scott Kelly, space station commander.

One of the most notable items brought to orbit, a humanoid robot named Robonaut 2, probably won't be unpacked until after Discovery leaves, however. On Thursday, during a call to the station, President Barack Obama encouraged the crew to set Robonaut free.

"We've all been voting to move up getting him out of the box," said Katie Coleman, a station astronaut. "We're all pretty sure we hear some scratching from inside there."

Work on other planets?

Robonaut is an experimental device that will perform some basic tasks around the station and provide information to engineers about the challenge of developing robots that work in the environment of space. More advanced versions will likely be sent outside the station and, possibly, to other planets.

All the same, while they're working to prepare the future way forward for NASA, the crew of Discovery can't help but think about the end of the shuttle era. After Discovery returns to Earth just two flights remain, those of Endeavour and Atlantis.

That has Stott thinking about next Wednesday.

"It's going to be hard to walk away from Discovery on that day," she said.